Lactic acidosis syndrome is a complication of therapy with nucleoside reverse transcriptase inhibitor (NRTI) drugs. Measurement of lactate levels is important in clinical trials of antiretroviral treatments. The preferred collection method for determining plasma lactate is with NaF/KOx tubes, while a number of ACTG studies have stored blood in ethylenediaminetetraacetic acid (EDTA) tubes. This study will analyze lactate in samples collected and stored in NaF/KOx and EDTA tubes to investigate the reliability of lactate concentrations over time.